Resolvers are used extensively in motor control systems to provide angular position and velocity feedback. Generally, the resolver includes a primary winding energized from a carrier frequency source and includes a pair of relatively rotatable secondary windings positioned in a quadrature relationship. The secondary windings provide signals at the carrier frequency with an amplitude envelope corresponding to sine .theta. and cosine .theta., respectively, where .theta. is the angular position of the resolver shaft. The signals are then demodulated to remove the carrier and recover the amplitude envelope. The sine .theta. and cosine .theta. signal values are then processed to determine the angle .theta. which is the shaft position. The advantages of the resolver over other types of position indicators are relatively low cost, virtually unlimited resolution provided by the continuous analog signals and immunity from most stress and vibration problems.
Resolver to digital processors are available which convert resolver analog signals to a digital values representing the angular shaft position .theta.. Such processors usually provide a digital shaft angle representation and an analog velocity indication. Among the disadvantages of the prior processes are relatively poor digital resolution (particularly at high speeds) and limited tracking error tolerance. The phase relationship between the modulating source and the modulated feedback to provide the sine and cosine values is critical and often requires special cables. Also, such systems have limited noise rejection.